The present invention relates to a disposable hypodermic syringe comprising a barrel the front end of which is equipped with a firmly connected hypodermic needle or with a gland for fixing a hypodermic needle, in which barrel a tightly fitting piston can be displaced by means of a piston rod.
Various kinds of disposable syringes are known. Most frequently used are disposable syringes having a barrel made of a thermoplastic material. However, in some instances disposable syringes having a barrel made of glass are preferred in spite of the higher price. For both types of barrels the piston can be made of a suitable type of elastomer and the piston can be equipped with a piston rod made of a thermoplastic material. Alternatively the piston and the piston rod can be made in one piece of a suitable thermoplastic material possibly to be equipped with a gasket made of a suitable elastomer. A common feature with these disposable syringes is that the piston may slip out of the barrel during the filling of the syringe so that the contents will be spilled or the sterility spoiled or both if the piston rod is pulled too strongly.
Some barrels made of thermoplastic materials do have a circumferential bead on the inside of the rear part of the barrel in order to reduce the risk that the piston may slip out. However, since the bead is an integral part of the barrel its size must be moderate enough to allow the passage of the piston during the assembly of the syringe. Therefore, the bead cannot effectively prevent the piston from being pulled out of the barrel.
From DE patent application publication No. 2,329,390 it is known to weld or otherwise fix an inwardly protroding flange to the rear end of the barrel of a hypodermic syringe after the piston has been placed in the barrel. The flange prevents the piston from being pulled out of the barrel. During the welding process the flange and the barrel must be carefully positioned relative to each other. For barrels of different diameters different flanges have to be produced.
The purpose of the present invention is to provide a disposable syringe which eliminates the above-mentioned drawbacks connected with the disposable syringes according to the known art.